Power plant operators and boiler manufacturers recognized early on that to improve the efficiency of steam generators (both fossil and nuclear), it was useful to adopt regenerative feedwater heating. Essentially, steam is extracted from the steam turbines to preheat the boiler/reactor feedwater before it is introduced into the economizer of a boiler or directly into a steam generator/reactor. The heating of the feedwater occurs in, naturally enough, feedwater heaters. Steam is used to heat the feedwater inside the feedwater heater tubing to impart a portion of the steam's latent heat to the water. Water temperatures from about 100.degree.-650.degree. F. (37.7.degree.-343.3.degree. C.) and pressures up to 5200 psi (358.53 MPa) are not uncommon. Moreover, advances designs are now contemplating pressures up to 7200 psi (496.42 MPa) and 700.degree. F. (371.1.degree. C.).
Currently, steels (carbon and stainless) and sometimes nickel-copper alloys are utilized in feedwater heaters. Although the feedwater is treated to remove chemicals and other impurities, corrosion of the tubing may still occur.
Superalloys are often difficult to form into tubes due to their high work hardening rates. High copper-containing materials are generally frowned upon since copper and corrosion products are believed to deposit on boiler tubes and may be carried over into the steam. These undesirable entrained products may enter into the turbines resulting in lower efficiencies. Indeed, operators wish to eliminate all possible copper pickup in the steam because of fouling and the resulting loss of efficiency of the turbine blades when the copper plates out of the steam. It is also believed that the copper deposits may set up local galvanic cells with the ferrous alloys thereby causing additional corrosion. Operators wish to stay away from nickel-copper alloys which otherwise display better chemical and physical properties than the other alloys. However, the substitution of low carbon or stainless steels for the nickel-copper alloys currently available are not always satisfactory since these materials do not have the requisite corrosion resistance, stress corrosion cracking resistance or strength. This leads to high maintenance costs. Moreover, in the case of carbon steels, undesirably short lifetimes of three to eight years have been reported. Contrast this state of affairs with an expected service life in excess of twenty years. Accordingly, power plant operators are in a quandry: steels corride; high alloys are costly; and the nickel-copper alloys contain high quantities of copper.
In addition, petrochemical installations employing piping and tubing are often subject to polythionic acid (H.sub.2 S.sub.x O.sub.6) cracking. Intergranular cracking is believed to be caused by the depletion of chromium along the grain boundaries.
The upshot of all this is that an alloy exhibiting the requisite physical and chemical characteristics should also have a low workability rate. In this fashion, the amount of time and effort needed to process the resultant tubing is greatly reduced. Indeed, many suitable alloys displaying good corrosion characteristics, because of their high work hardening rates, require additional thermomechanical processing steps in order to manufacture suitable tubing.
For example, now expired U.S. Pat. No. 3,168,397 marketed as alloy 20Cb-3.RTM. (a trademark of Carpenter Technology Corp.) and U.S. Pat. No. 4,201,574 (identified as alloy SCR-3) have been suggested as suitable tubing alloys. A paper entitled "Development of New Alloy SCR-3 Resistant to Stress Corrosion Cracking in High Temperature High Pressure Water" by M. Kowaka, H. Fujikawa, and T. Kobayashi, Golden Gate Metals and Welding Conference, San Francisco 1979, further explains alloy SCR-3. These austenitic alloys, is the as cold worked condition, have higher tensile properties and lower ductility than the instant age-hardenable alloy that necessitate additional processing steps. Test data, included herein, indicate that the instant alloy is more easily fabricated into tubular shapes, thereforee necessitating lower manufacturing costs.
These austenitic alloys are supplied in the annealed condition at relatively low tensile strengths in order to resist stress corrosion cracking and to be capable of making small radii tube bends. On the other hand, the instant age hardenable alloy may be cold worked to greater levels of cold work and thereby eliminating expensive processing steps as will be shown later. Then by nature of the age-hardening capability the instant alloy may be heat treated to a higher level of tensile strength and still resist stress corrosion cracking and maintain adequate ductility to make tight U-bends. Tubing exhibited 25% tensile elongation is marginal and tubing with 18% elongation or less nearly always fails small radii bending.
It is apparent that there is a need for a resonable cost, age-hardenable alloy that exhibits corrosion resistance, strength and formability properties suitable for feedwater heaters, chemical and petrochemical installations and other similar applications.